Control of Condensation in the Modern Pitched Roof

house roof and condensation

THE CONTROL OF condensation in the roof space can be a particular challenge in the UK as it has one of the oldest housing stocks in Europe.

With around one house in five built before 1919, the majority of UK homes are houses or bungalows (as opposed to flats). Nearly all homes are traditionally-built masonry or timber constructions with tiled or slated pitched roofs.

As we strive to improve the energy efficiency of our homes, greater levels of thermal insulation reduces the average temperatures within the roof structure.  Warm moist air, generated within the living spaces, finds its way through the ceiling into the cold roof space.

What do we define as the roof space?  The roof space is the ‘cold’ area between the insulation and the roof underlay.  In a typical home, where the insulation is laid over the ceiling joists, the roofspace is the loft.

Where the loft is a living space; i.e. the insulation is laid parallel with the rafters (typically under and/or between the rafters). This means the roof space is the much smaller gap between the insulation and underlay.

Condensation and Water vapour

The ability of air to hold moisture reduces as it cools, so it deposits the moisture onto cold surfaces in the form of condensation.  For example, in a two-storey house with a floor plan of 100m2, there is a total of 420 cu metres of air. At 200C, this could potentially hold up to 8.4 litres of water vapour.

Flow of vapour causing condensation into roof space
Fig 1

If this warm air passes through the ceiling into the roof space and cools down to 100C, it can then only hold 3.4 litres of water vapour.  So 5 litres of water will be deposited somewhere if it is not allowed to escape from the roof space – fig 1.  It may not be as dramatic as that in practice, but it illustrates the potential risks.

More insulation

Flow of air into roof space avoiding condensation
Fig 2

So why do we have a potential problem?  Until the late 1970’s we predominantly used open cavity construction in the UK.  Air bricks were installed at low level to allow air to pass into the space under the timber floor and into the cavity walls.  The tops of the cavity walls were open and air was able to flow to and from the cavities into the roof space.  This all meant that cold air passed all around the outside of the living areas – fig 2.  Of course, because the construction was so air open generally, thermal performance was extremely poor. Little, or often no, insulation was installed. But, the upside was, condensation was not usually a problem!

Since the early 1980’s we have been closing cavities at the top and using cavity liners to restrict air movement into the cavities.  At the same time, we have steadily increased the level of insulation in the roof and now include it in the cavity walls. More recently, we started to better understand the thermal benefits of more air-tight construction.  This all means that controlled ventilation of both the living space and structure becomes all the more important.

Condensation and Breathable Membranes

Bringing older houses up to modern thermal requirements is often relatively easy with the addition of double glazing, cavity wall insulation, extra loft insulation etc. However, it is far more difficult, if not impossible, to create effective vapour barriers and well-sealed ceilings to prevent the moisture-laden air from reaching the roof space.

The switch to vapour-permeable and air-permeable roofing underlays has been greatly beneficial. It helps prevent harmful levels of condensation from building up in the roofspace. However, it is important to use products correctly, in accordance with the guidance in BS 5250, and with the information in the underlay’s accreditation certificate.

Essentially, in simple terms, there are two ways we can control the risk of condensation build up in the roofspace. Firstly, we can prevent water vapour from reaching the loft space in the first place. Alternatively, we can remove it once it gets there before it has chance to build up to harmful levels.

Keeping water vapour out of the roof space

To prevent the water vapour passing from the living space into the cold roof space, it is necessary to install effective vapour barriers.  It is difficult, if not impossible to construct  a totally air-tight ceiling, so British Standard BS 9250 gives guidance on minimising air leakage through junctions and penetrations such as light fittings, loft hatches etc to create a ‘well sealed’ ceiling.

We now have well-documented methods for achieving this in new buildings, though it is usually difficult in existing buildings. Greater energy efficiency is achieved if air leakage through the ceiling is prevented. This is true of reducing the risk of condensation also. However, where this is not practical we must use adequate ventilation to remove the water vapour from the roof space.

Eaves and ridge ventilation in roof space to dispel condensation
Fig 3

Breather membranes

BS 5250 sets out the minimum requirements for roofspace ventilation.  For example, in an average house with a cold loft, there should be a minimum of a 7mm continuous ventilation gap at eaves when using a vapour permeable underlay with a normal unsealed ceiling. With all the benefits of dry fix systems well documented, it also makes perfect sense to supplement eaves ventilation with high-level ventilation using a dry ridge system – see fig 3.

Eaves to eaves ventilation works well in theory. However, it relies on external air movement and may not work so well in practice. This can particularly be the case if the house is too close to adjacent houses. Frequently, homeowners will fill the loft with items which block the air flow.

Another, common example is where a vapour permeable underlay is in use and the insulation is located parallel with the rafters. If an effective air and vapour control layer and well-sealed ceiling can be guaranteed, then there is no requirement for roofspace ventilation. Otherwise, there should be eaves to ridge ventilation. This creates clear airways in all rafter voids between the insulation and underlay.

What to consider

It is worth considering when designing a building that occupants don’t always use the building in the way designers intend. This means erring on the side of caution and providing robust solutions. For example, a family with several young children may generate far more condensation than a single person. This can push ventilation systems beyond their limits, particularly in winter.

Building Regulations and BS 5250 recognise that temporary condensation may occur during adverse climatic and internal conditions. (For example, very cold outside with little or no air movement, warm indoors with no windows open).  It is common to see temporary overloads of condensation appearing on the underlay. This dissipates within a few days with no harm done – usually during very cold but still weather conditions. However, temporary condensation cannot be severe enough to cause damp or staining on internal surfaces or damage to the structure.

Summary

  • In newbuild homes we must strive to minimise air leakage through ceilings. This is done with efficient well sealed ceilings and air and vapour control layers
  • For older properties we must accept that air leakage happens. This means adequately ventilating the roof space is necessary to prevent harmful condensation forming
  • In extremes of cold/still air condensation ‘overload’ may occur. This can happen even where the roofspace ventilation complies with the guidance given in BS 5250. So supplement eaves ventilation with high level ventilation for greater efficiency

>>Read more advice on condensation

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